Method of making permanent facsimile copies



Nov. 4, 1958 B. L. CLARK ErAL 2,859,351

METHOD 0F` MAKING PERMANENT FACSIMILE COPIES Filed oct. 25.` 1954 /fedf-fenf//e cnaf/27g. fupparmg baie.

MVM

, 2,859,351 Patented Nov. 4, 1958 METHOD OF MAKING PERMANENT FACSIMILE COPIES Application October 25, 1954, Serial No. 464,212

4 Claims. (Cl. Z50-65) This invention is concerned with heat-sensitive thermographic duplicator sheet material or copying-paper useful in preparing copies of printed matter or the like, and more specically with thermographic copying-paper .in which particles of fusible material provide a means of obtaining a permanently visible change on the application of heat.

This application is a continuation-in-part of our copending application Serial No. 209,062, now U. S. Pat. No. 2,710,263, which in turn is based on lapplication Serial No. 747,340, filed May 10, 1947 and now abandoned.

Heat-sensitive copying-papers of the type here contemplated are of particular utility in making copies of graphic subject-matter such as printing, drawings, diagrams, pictures, etc. by methods to be described. Such methods involve the irradiation of the graphic subject.

matter with intense radiant energy of proper wave-length, the resultant formation of an elevated-temperature pattern corresponding to the graphic matter irradiated, and the utilization of such elevated-temperature pattern in directly producing a corresponding permanently visible pattern in the copying-paper.

Radiant energy suitable for providing the required elevated-temperature pattern may be obtained from a number of sources, including electrically-heated incandescent laments, electric arcs, and focused sunlight. Electrically-heated incandescent filament lamps are readily available, simple to operate, and safe to use. For copying typewritten letters or the like, a 3000-Watt tubular lamp with a coiled filament inches in length has been found eminently suitable. The Vlamp is located in -a reector which concentrates the light in a narrow line, and the line of light is moved across the typewritten sheet to provide the required brief intense irradiation. The apparatus and the method of operation are described and claimed in the copending application of Carl S. Miller, Serial No. 180,617, tiled August 2l, 1950, now U. S. Pat. No. 2,740,895, as a continuation-in-part of Serial No. 747,338, iled May l0, 1947, now U. S. Pat. No. 2,740,896.

In our-novel heat-changeable copying-paper, heat-sensitivity is obtained by employing a layer of inherently transparent fusible material in the form of light-dispersing particles. The fusion or melting of these particles results in a change in the optical properties of the heated portion, and makes possible the reproduction of typewritten messages or the like by the methods hereinabove indicated. v

Other workers have designed and produced heat-sensitive sheet materials, for a variety of purposes. One common purpose is for the tracing of lines or iigures with a heated stylus; the pressure of the stylus displaces the heated surface layer to produce a visible trace. Another application is in determining the temperature of a surface, in which case a large quantity of heat energyfis available Iand only one surface of the testsheet is ordifor certain other applications.

narily in contact with another surface. -In many prior art heat-sensitive sheet materials, the sheet is not transparent to infra-red radiation, hence direct exposure to high-intensity infra-red would result in absorption of.

such energy, conversion to heat, and activation of the entire heat-sensitive sheet. Other known products when heated become sticky, or the sensitive layer is weakened and splits or osets, or the visible change produced on heating is not permanent. We have found that all such products are unsuitable, in one or more particulars, for application as heat-sensitive copying-papers in accordance with the methods hereinabove indicated.

It is therefore a primary object of this invention to provide heat-sensitive thermographic copying-papers which avoid these and other deciencies, for the indicated purposes, of prior art heat-sensitive sheet materials and which are adapted for use in the reproductionof graphic subject-matter, such as typewritten messages, by methods involving brief intense irradiation of such message while in heat-conductive relationship to the copying-paper as more fully described in the applications of Carl S. Miller, previously mentioned. A particular object is the provision of such a copying-paper which is susceptible to front-printing methods wherein the irl radiation of the typewritten message occurs through the copying-paper, which in such cases must be capable of transmitting the heat-producing radiation. Other objectsy will be pointed out or will become apparent on consid-4 eration of the specification taken as a whole.

A'Figures 1 4 of the accompanying vdrawing are diagramor uncolored. In Figure 2, the non-transparent heatsensitive layer 21, carried by the supporting base'20, is further protected by a transparent surface layer 22. Figure 3, a color layer 33 is interposed between the uncolored base 30 and the non-transparent heat-sensitive layer 31. The combination structure of Figure 4 includes a base 40, a non-transparent heat-sensitive layer 41, an interposed color layer 43, and a protective surface layer 42.

Where terms such as transparent and non-transparent are employed, it is to be understood that they refer to visible light. used in preference to opaque, since the coatings do transmit some light, even though such light is highly diffused. The term infra-red-transmitting is used to designate materials which permit the passage of the invisible infra-red radiation or of other high-intensity high-` energy radiation which is absorbed by the printed char.-

ing-paperagainst the back or unprinted surface of the f document, preferably with the heat-sensitive surface toward Ythe same, and then irradiating the printed face. of`

the document. Absorption of the radiation by the printed characters results in generation of heat which is then conducted through the thin paper to the heat-sensitive surface, producing a visible change therein. This f back-printing method is thus capable of producing direct reproductions where the original is printed 'on a satisfactorily thin and heat-conductive paper or othermaterial, but is not so satisfactory for heavy book paper, or for thin papers heavily printed on both. surfaces, or

In the structure of Figure l, a non-transparent. heat-sensitive layer 11 is carried by a supporting' base' 10, which may be transparent or non-transparent,.colored Inv l The term non-transparent is In frontrintin however as hereinbefore mentioned, the effective radiation first passes through the non-transparent heat-sensitive copying-paper, which must therefore be infrared-transmitting. The heat pattern resulting from absorption of this radiation byl tlie inkedv letters ofthe printed surfacey causes a' visible change 'in the copying-paper, resulting in the reproduction' of the; printed message. This! Vreproduction` is'a dirlectv` copy; of the original when viewed from the sideof the'A sheet-at which the radiation was-initially directed.' 'I-he-'clarityj and contrast ofi the resulting-copyV produced by 'frontprinting is thus independent of the thickness of the original printed' page. Y

VThus we havey found that certain requirementsy mustbemet by the-non-transp'arent, heat-sensitive coating as4 Well as by the copying-paper as a whole in order to assure the production of Vpermanently visible, clear and sharp copiesV by methods here contemplated. The following specific examples will illustrate these requirements, but are not to be construed as limiting the scope-f of the invention, since manyY other modicationswillbecome apparentfon consideration of the disclosures here made.` Y

Example l Paper inl continuous sheet formis coated with a black undercoat,l and 'subsequently withY a coating consisting essentially of an; inherently transparent fusible material int'thefformof a non-transparent light-scattering particulate layer.` A, further thin transparentA protectiveY surface coating may be applied if desired.. `The separate coatings in solution or dispersion form areV applied in controlled thickness, as by means of a spreader knife or-bar, and the volatileA solvent removed by evaporation.

The black undercoat as applied consists of a mixture ofV 50 grams` of al uniform suspension of 200v grams of Nigiosine dyein-.one liter. of heptane, and grams of al 5%. solution of .latex crepe rubber in heptane. The rubberacts as a binder for the particles of dye. Suitable antioxidants, vulcanizing agents, orotheringredientscontributing tov improved; agingr life or other propertiesl of l the rubber may be added if desired. The coatingknife is setat anorice of 2 mils (.002 inch) to provide a thin. but effective substantially opaque black background or undercoat. Heat may be applied to remove the solvent and to cure the binder.

The non-transparent` heat-sensitive layer is prepared by coating a mixture of. 50 grams: of asuspension of 100 grams of cadmium stearate in 500 milliliters of heptane, and 10 gramsY of the 5% rubber solution, at a coating oric'e of"3.5 mils. Thecadmium stearate,vwhich is insoluble in heptane,. is suspended in nely divided particulate form in the volatile vehicle by prolonged milling in a ball`mill.V This. coating is dried at a temperature less than the melting-point of the cadmiumstearate; forA example, at normal room temperature. The cadmium stearate layer remains as a Well-bonded non-transparent light-diffusing crystalline or discontinuous surface layer.

Cadmium stearate and rubber are mutually compatible, as shown by thedifferent melting-points ofthe cadmium stearate alone and of the mixtureA after'blendingat temperatures somewhat above themeltingpoint of the cadmium stearate. For example, heating together 10 parts of cadmium stearate melting at 98.5-102 C. `with0.5l part of rubber for. minutes at 14041609 C. provided a compatiblel blend having amelting-point of128-1-32u C.

In this and similar cases the melting point is conveniently determined on a Fishery melting-point apparatus, the samples being placedon a thin glass support-audiobserved through a binocularlmicros'cope.y The meltingpoint is taken as that temperature. at which the powderV or solidied droplet liquies s'uicientlyvto flow out` as a smooth layer. The rate of temperature rise is; about onedegree per minute, andin nofcase. greater thanA 'three degrees per minute. Y

While the structure above,provided;producesiexcellent vcopies without offsetting or splitting of the heat-sensitive layer, afurther thin surface coating or sizing of a 5%r solution of cellulose acetatel in acetone, appliedl at an orifice of 1.5 mils and dried at room temperature, provides a desirable additional degree of surface protection for the fusible layer. The acetone is a non-solvent for the components of the previously applied layer, and the solution may therefore be applied without appreciably transparentizing the heat-sensitive coating.

Heating theY sheet. material to or above approximately the melting-pointof cadmiumstearate transparentizes the heat-sensitive. layer and. allows the black undercoat to become permanently Visible at the heated area.

The` cadmium stearatewas prepared by reaction in dilute aqueous solution of one mol of cadmium acetate with two mols ofV sodium4 stearate, prepared from commercial triple-pressed stearic acid and sodium hydroxide.

Thepprecipitated cadmiumstearate was recovered by lil-` tration, washedwith'water, and .dried at room temperature.V It hadv armelting point of approximately 100 C.,

and; in thinjtl'lms was clear` andI transparent.

SincefNigrosine dye-ishighlyabsorbent to infrared, this sheet'was notsuitable for frontprinting. The same was' true Iof'copying-paper in which carbon blacky was employed as the coloring agent in the color layer. However, suchv sheetsl provide clear and distinct copies of typewritten letters and;` the like by theback-printing process as previously described, in which the heat-sensitive side of thecopying-sheet is pressed against the back surface of a thin printed page whichis then irradiated on the printed orlfront side.,

Example 2 In this example,` aY thin but intensely colored blue background or undercoat is provided by coating the paper or other base. sheetA material with a mixture of 40`V grams'` of an ultramarine suspension and 20 gramsiof a 10% solution of vvcelluloseacetate in acetone, applied at p The ultramarine suspension-.isprepared by milling 100-grams Vof the pigment into' an- Vinitial-thickness of Z'mils;

200 ml. of acetone in a ball mill.

A. second coating is then applied,-consisting of a mixtureof.` SO-gramsof` a suspension caf-200` grams of lead" laurate in-800 ml. of heptane, and 10 grams of a 5%A rubber. solution'in heptane as used in Example 1. The

coating as appliedis 4 mils thick. It is dried at roomY temperature. The dried residue is in the form of a rough; light-dispersive coating, of-just sutiicient thickness to provide. adequate light-dilusing ability and to obtain desired contrast between the coating-and the blue background.

Alnal protective coating of 1.5 mils of a 5% solutionY of cellulose acetate in acetoneis desirably then applied and dried at room temperature.

TheY resultingV heat-sensitive copying-paper appearsV bluish-white on the coated .surface and; changes rapidly to an intenseblue color at areas heated -to or somewhatof` 800vparts of sodium hydroxide in 10,000-parts of Water was prepared. The sodium hydroxide solution wasA added slowly to the fatty acidfmixture to form a dilute` solution of. sodium laurate soap. A solution of 3794 partsof'lead acetateftrihydrate in30,000A parts of water'` The entire mixture The lead was then added slowly with stirring.

were washedand dried. The salt prepared from highly purified lauric acidappeared'to be fully equivalent to that preparedY from afcommercialproduct stated to contain about laurio acid., The melting point of thisy waxymaterial. was about 87 ff C. i Lead Vlaurate-L andl crepe rubber have.y been shown t Specifically, 4060 be fully compatible by means of melting-point tests as described in connection with thev cadmium stearate and rubber of Example 1. The melting-point of the heatblended mixture is increased over that of the waxy material alone.

The thin ultramarine blue color coat of Example 2 is infra-red-transmitting; and When it is combined with a transparent and infra-red-transmitting backing such as varnished paper, glassine paper, cellulosic films or the like and the sheet coated with the heat-sensitive coating as described, there is provided a structure which is suitable for front-printing as well as back-printing.

Analogous results may be obtained with other color layers. For example, toluidine toner may replace the ultramarine blue to provide a copying-paper which gives red-colored copy against a white or slightly reddish background.

Coatings fusing at temperatures much lower than about 60 C. will obviously be unstable under many customary conditions of storage. On the other hand, for commercial copying operations employing procedures hereinbefore described, the maximum temperature obtainable has been found to be about 120 or 125 C., and we much prefer to operate well below this range because of the heavy power requirements, the short life of the incandescent filament, the possible deterioration of the printed original, and for other reasons. Hence our preferred temperature range is about 60-l15 C., i. e. the particles of normally transparent stable organic solid fusible material should melt without appreciable Volatilization or decomposition at a temperature within the range of about 60-115" C. to a liquid form having good wetting properties toward the infusible binder.

Where a single fusible material has too sharp a melting point for best results in terms of the clarity and detail of the reproduction, mixtures of two or more fusible materials frequently offer advantages.

A copying-paper comprising a blue undercoat and a lead palmitate non-transparent fusible layer, and produced as described under Example 2, supra, was found to have a conversion range of 8-10 C. with full conversion at about 102 C. The paper was used to copy a typewritten message. The letters appeared dark blue against a clean white background, but individual letters were observed to be slightly blurred. When half of the lead palmitate was replaced by lead laurate, the conversion range was increased to l2-l4 C. ending at about 90 C. The individual copied letters were somewhat sharper in detail, but the background was slightly.

blurred in some areas. Substitution of lead laurate for all of the lead palmitate increased the conversion range to l9-21 C. vwith full conversion at about 97 C., and gave good detail but caused further darkening or blurring of the background. All of the copies were easily readable.

Substitution of lead caprylate in the above sheet gave -a product having a conversion range of 28-30 C., with full conversion at about 99 C. While copy produced with this sheet could still be deciphered, the reproduced letters showed so little contrast in relation to the background that the product could not be considered a commercially satisfactory copying-paper.

The above two specific examples and indicated variations employ quite small proportions of infusible binder with the fusible particulate solids on which the copying process largely depends. Somewhat increased amounts of binder over those illustrated have been successfully applied in similar compositions; for example up to about 10 parts of ethyl cellulose has been combined with correspondingly about 90 parts of various waxy or other fusible particles to produce non-transparent, heat-sensitive coatings which transparentize at approximately the melting-point of the fusible material. When an attempt is made to use much larger proportions of binder, formulations and methods such as are described in Examples 1 and 2 are found frequently to'produce substantially transparentcoatings, or at least coatings which do .not

Example 3 v Parts by Weight Hydrogenated fatty oil wax, M. P.

65 C. Cornelowax No. 1469) 11.77 NitrocelluloseV (typeRR, 125 sec. viscosity) 4.20 Ethyl cellulose (type N, 50 c. p. s. viscosity) 0.70 Dioctyl phthalate 2.11 Acetone 45.50 Toluol n 35.90 Dissolve the nitrocellulose in a mixture of 18.20 parts of acetone and 5.60 parts of toluol. To this solution add the other ingredients, including the remainder of the'solvents, and reduce to a smooth suspension in a ball mill. Milling is continued until the insoluble wax has` been completely broken up into uniform microscopic particles and the suspension can be coated in a smooth, uniform, very thin layer. Where the total charge was lbs., milling for 8 hours in a 75-gallon porcelainlined ball mill with 1/2 inch porcelain balls was found to produce the desired suspension.

Apply the suspension as a uniform smooth layer to thin (22 lbs. per ream) clear transparent glassine, in van amount suicient to provide a dry coating weight of 2 grains per 24 sq. in., anddry at 30% relative humidity and 80 F. The resulting coating is white and nontransparent, and the structurecorresponds to that shown v in Figure 1 of the drawing. It is adaptable both to frontprinting and to back-printing techniques. The Wax and binder components are compatible, hence the copy produced is permanent, being visible either as a transparency or when held against a distinctively colored background.V For example, colored glassine may be substituted for the clear transparent sheet to provide a colored base or background. y The fusible waxy component of the heat-sensitive coating of Example 3 may readily be extracted, without altering the structure of the infusible binder stratum, by means of suitable selective solvents. When this is done, it is found that the binder remains in the form of a White, light-diffusing and non-transparent, porous self-Supporting web. Such a web may be locally transparentized by impregnating it with a drop of melted Wax; the spot remains transparent on cooling. Reducing the proportion of fusible material in the formula of Example 3 tends to raise'and broaden the ternperature range at which conversion of the non-transparent layer to the transparent form is obtained. In this example the ethyl cellulose serves to reinforce the coating but does not contribute to the light-diffusing properties of the coating. At less than about two parts of the fusible material to one of nitrocellulose the coating will not transparentize until heated well above the melting-point of the Cornelowax itself. We therefore prefer to add an amount of the latter such that the conversion temperature of the sheet is approximately the same as the melting point of the fusible material, and to adjust the conversion temperature by selecting a fusible material having the desired melting point. Hydrogenated castor oil wax (Opalwax), for example, has a melting-point of C. and is quite satisfactory; and other waxes or waxy materials as Well as other fusible materials of other melt-` ing points within the desired range and which are other'- wise suitable for our purposes have already been indicated. Where soft fusible materials -such as waxes are used, increasing the proportion of wax much beyond about a 6: 1v

ratio makes the coatingsusceptible to abrasion; since waxv A liquid coating composition-Was prepared from the following ingredients by ball milling as in Example 3.

. Parts by weight Hydrogenated' fatty oil wax, M. P. 65 C 1073.7 Nitrocellulose 382.0 Ethyl cellulose 62.4 Dioctyl phthalate 224.7 Acetone 5800 Toluol 2460 The composition was coated onthin (20-30 lb.) clear transparent glassineat -a coating weight, after drying, of approximately 3 grainsV per 24 sq. in. The sheet provided sharp, clear, permanently transparent copies -of typewritten originals under front-printing thermocopying conditions as herein described. Conversion of the nou-transparent heat-sensitive waxy layer to the transparent condition occurred at about 65 C. and within a range of about 2 C. The non-transparent binder stratum was somewhat less porous than that of Example 3 when coated under identical conditions, or approximately equal in porosity when coated under conditions of higher humidity.

A preferred alternative formula for use in making the heat-sensitive layer of the example, and with which highly effective heat-sensitive copying-paper has been prepared' in 'extensive comercial quantities, consists of 1910 parts of Cornelowax No. 1469, 177 parts of ethyl cellulose, 62 parts of nitrocellulose, and 239 parts of butylstearate, dissolved or dispersed in a solventV mixture of 7l30vparts of acetone and 480 parts of toluene. The wax is dispersed in aV solution of part of the nitrocellulose in a portion of the solvent by prolonged ball-milling, the remaining ingredients then being added and ball-milled suflciently to produce a smooth uniform dispersion.

In both formulas the binder material and fusible material may be shown to be compatible by methods described in connection with Example l.

In all of these constructions, the non-transparent heatsensitive layer is permanently visibly altered when heated to or somewhat above the melting point of the fusible particles of which it is comprised. The exact temperature at which the visual effect is obtained is found to be a function not only of the melting point of the fusible material but also is a function of the relative amount of such material and the presence or absence of various modifying agents. It is also a function of the test procedure employcd, as will be further pointed out.

Example A series of coatings was prepared from Cornelowax No. 1469 and nitrocellulose in different ratios, applied from a mixture of acetone and toluene; and the temperatures at which the dried light-diffusing coatings became clear and transparent were determined. At one part of wax to one of nitrocellulose, the coating did not become completely transparent until about 120 C. As the ratio of wax to nitrocellulose was increased to 2:1, 3:1, 4:1, 5:1, and 6:1, the temperature required for transparency became, respectively, 110 C., 95 C., 83 C., 67 C., and 67 C. At higher ratios than six of the wax to one of the nitrocellulose, the coating was found to be undesirably soft for some applications, particularly where the sheet Was subjected to scufng or abrasion. The specific temperature required with each wax: binder ratio is somewhat dependent on the ratio of the two volatile components of .the coating composition. Increased amounts of toluene ordinarily require the presence .8* Y of somewhat larger amounts 'of the wax component in order to achieve full transparency at a comparable temperature. Omitting the toluene, on the other hand, results in a coating which. is undesirably close to the transparent condition immediately Vafter drying and without heating.

The above wax-binder'ratios and corresponding .trans-V parentizing temperatureswere allbased on identical -acetone-toluene ratios inV the coating composition.

The additionrof small proportions of plasticizers compatible withthe nitrocellulose, such as dioctyl phthalate,

reduced the amount of wax required to reach the mini-v mum transparentization temperature.

Where there was a tendency for the heated area to appear slightly foggy, presumably due to inetective wetting of thefbinder by themelted/Wax, a trace of plasticizer was found to improve such `wetting actionand to impart improved clarity to the transparentized area. Large amounts -of such `plasticizer produced a coating which was not suiciently light-diiusing to provide the desired degree of4 contrast. Y

Other plasticizers Vand modifiers may be selected to provide analogous .effects with other fusible materials and other binders.

'Example 6 Waxy polyethylene glycol (Carbowax 6000) was ball milledin acetone .at a concentration of 30% wax. Nitrocellulose sec.) was-dissolvedgin. a., mixture of 50 parts acetone and40 parts toluene to a concentration of 10%. A mixture of 20 parts of the wax dispersion and 40 parts of the nitrocellulose solution was coated on thin glassine at a wet thickness of 3 mils, and was dried at room temperature. The resulting light-diffusing coating could be permanently transparentized at `about 56 C. Thermocopies prepared with this copying-paper were readily visible when'the sheet was held against a dark background. However, improved results were obtained when a colored base for the heat-sensitive coating was employed. For example, the glassine could irst be given a rcolor coat of Diane blue in ethyl cellulose dissolved in a mixture of ll parts heptane and 77 parts toluene, with a trace of glacial acetic acid as a deocculant, or in a solution of rubber hydrochloride (Plio lite) in heptane. In place of Diane blue, other coloring agents such as Monastral blue, methyl violet, or Ponsol jade green have proven useful in such applications. These coloring agents are infrared-transmitting and hence the resulting sheet may be used for either front-printing or back-printing.

The polyethylene glycol particles could be easily extracted from the non-transparent coating with suitable solvent, leaving a non-transparent porous stratum of the binder. After the coating had been heated and transparentized, such a separation was no longer possible. This behavior is to be contrasted with that of coatings such as described in' Examples 3-5.

Carbowax 6000 melts at 59-6l C., whereas a mixture of 7.2 parts of the wax and 2.8 parts of nitrocellulose heated for 20 minutes at 80 C. with stirring and cooled to solid form was found to melt at 79.5-82.5 C.,

thus showing that the two components are mutually f compatible.

A particular advantage of our new paper lies in its ability to copy printed, typewritten yor other graphic material accurately and sharply under the peculiar conditions herein described and illustrated, i. e. in heat-conductive Contact with the intensely irradiated graphic subject-matter.

A possible explanation of the previous lack of a copy- Y ing-paper capable of producing permanently visible, sharp and clear images in a fusible layer by the novel method here employed lies in the lack of appreciation, in the prior art, of the effect of various factors inuencing such reproduction. The most important ,of these factors will now be mentioned.

' While many sources of radiant energy may be used inl our process, as previously noted, the most convenient source is the incandescent elctrically-heated filament in a suitable reflector. The intensity of the radiation from such source is limited by the melting-point of the material, usually tungsten, of which the filament is constructed. Our copying-paper must therefore be designed to-vprint at temperatures which it is possible to reach by irradiation of the graphic material from such source of radiant energy.

Radiation of the infra-red absorpu've inked portion of a printed page causes a rapid -rise in temperature at such points; radiation of the surrounding ink-free areas also causes a less pronounced, but nevertheless denite, heating eect. Prolonged irradiation consequently tends to produce an optically observable effect over the entire surface of the copying-paper, and is to be avoided. The copying-paper must therefore be capable of rapid printing.

For copying both fine detail and-massive or blocky areas,'.we have found that an appreciable interval, measured as hereinafter described, between start and completion of fusion of the fusible layer is highly desirable. When this interval is too small, either the fine lines of the graphic material are not copied, or the heavier areas are badly blurred. The background area, however, remains fully non-transparent and provides a high degree of contrast. On the contrary, when the reaction interval is too extended, both tine lines and blocky portions are printed, but the entire sheet is darkened and the contrast between printed and unprinted areas is reduced.

'At temperatures corresponding closely with the ambient temperature, the temperature interval may be` quite small, less than a degree centigrade being found effective in the neighborhood of 60 C. At higher temperatures, a more extended interval has been found necessary in order to provide clear and distinct copies of subjectmatter containing both line detail and heavy massive areas. Thus at 100 C. an interval of about 5-15 C. is preferred, and good results have been secured with papers which showed an interval of as high as about 25 C. when tested as herein indicated. However, at higher intervals, the reproduced letters showed very little contrast in relation to the background.

We have found that a change in any one of the components of our composite sheet material may have a considerable effect on the ability of the material to produce acceptable copies. For example, differences have been noted in the copying characteristics of the sheet ou substitution of a particularly dense backing such as heavy parchment-paper for the paper of Example 1; or on the application of an increased thickness `of undercoat; or on the use of a dyed paper backing in place of a pigmented undercoat on a paper support. Differences in the purity and degree of dispersion of the fusible material, and in the amount and kind of binder, and in the thickness of the fusible layer, have produced noticeable differences in the copying characteristics of the sheet, as has the amount of film-forming material applied as the protective surface coating. It has therefore been found impossible to define accurately the requirements of these 'copying-papers in terms of classes of components and proportions thereof.

We have, however, been successful in devising a test method for determining the suitability of specific sheet materials as copying-papers. In this method, a brass bar is heated at one end to establish a temperature gradient The conversion temperatures and aigs' of tfip--.

tures previously' enumerated herein have in each case been determined by means of the test method described. In many cases the temperature range within which the visible change occurs in the copying-paper, when so tested, is not the same as the melting point of the fusible material itself.r Thus in Example 5, a wax having a reported melting-point of 65 C.. was the sole fusible material in each ofthe several copying-'papers transparentizing at 67 C., 83 C., 95 C., etc. Furthermore the presence of traces or larger amounts of plasticizers or other modifiers frequently has an effect on the melting-point of the fusible particles as well as on the temperature of visible change of the `coating. Nevertheless we have -found that our normally transparent-stable organic fusible solid material should have a melting-point within the range of about 60-115 C. in order to be adapted to the production of our novel copying-paper.

It vwill be apparent from the foregoing that the temperature interval over which the visible change occurs,

- and its location on the temperature scale, is only partly throughout its length, and i-s pressed for l-2 seconds against the copying-paper supported on a rough-surfaced sheet of sponge rubber. Knowing the temperature at dierent points along the bar, it is then possible by inspection of the heated paper to determine the temperature at which the ysheet first starts to show a visible change, as well as the minimum temperature at which maximum visible change is obtained, and from these values to determine the conversion range.

a function of the true melting-point of the fusible compound, and depends also on the several other components of the copying-paper as well as on the test method employed.

Heat-sensitive copying-papers prepared according to Examples 1-5 were tested by the above method, with results as follows: f l

Temperature Interval for Visible Change comme:

`The amount and condition of the binder component of the heat-sensitive coating is of considerable significance to the copying ability of the sheet. The binder is infusible atfthe temperature of fusion of the fusible particles. The binder material must be transparent, at least in thin continuous lilms. It must have substantially the same refractive index as the fusible material, so that when combined -together in the heated coating the combination does not cause scattering of light but instead is clear and transparent.

The binder must be present in an amount su'icient to I hold the particles in place, but not to such an extent as to prevent the obtaining of a distinct visible change on hea-ting. Very small amounts of binder are quite effective, as shown in Examples 1 and 2, where the ratio of infusible binder to fusible particles is about 5 10 to 100. Much larger amounts, as used in Examples 3-5, may also be employed and are preferred in many instances.

4The binder may, as shown by the examples, be present in amounts of from about 5% to about 50% based on the total of binder plus fusible particles, and depending on -the structure of the binder stratum. About 5-10%' of binder is sufficient, as shown by Examples l and 2, to bond the fusible particles together and to the supporting base. A-t these low `proportions the transparent binder stratum is completely masked and the coating rendered non-transparent by the high percentage of fusible particles. At increased lproportions of binder, e. g. between about 15% and about 50%, some4 of the binder itself must contribute 4to the light-diffusing property of :the coating in 'order to-*lprovide a desirably non-transparent heat-sensitivel coating; At proportions much above 50% of binder, the amount' of fusible particles is insuicient to provide adequate visible contrast between the unheated and the heated areas.

The optical change on which the copying process dependslis in every case the resultof a permanent transi I1 parentization phenomenon caused by fusion of the fusible particles. Thus in the 4structure of Figures 1 and 2 of the drawing, the initially'.non-transparentheat-sensitive.

ing initiallyobs'c-ures ay color. layer, which thenbecomes,` visible' throughthe coating-when. the` same 'is@transparfV entized y011 hea-ting.

To, provide forl suchtransparentization, it is necessary that a number of requirements, alreadyindicated to some extent, be-fuliilled. The; material ,of which the fusible;

particles are composed must itselfbe transparent. The particles must vbeY capable-of-melting and-fusing within the overall temperaturemangeobtainable in the thermographic process. The fusible material must neither decompose nor volatilizewithinthese temperatures. Other materials which might cause degradation of the supporting baseor the binder or other components, -or which are toxic to handle, or which are unstable when exposed to actinic light or atmospheric moisture or oxygen, might be temporarily adaptable but lwould be of no commercial interest in producing` heat-sensitive copying-papers for the permanent reproduction of printed, typed, and similar graphic matter, and such unstable materials. are specilically excluded.

The fused material ymust be capable of wetting the nonfusible binder, and the two should have substantially the same refractive index. The interfacial relationships between binder and fusible material may of course be controlled to some extent by addition of small amount-s of soluble wetting agents, plasticizers, or the like. Thusthe dioctyl phthalate.ofExample 3 is observed to contribute to the transparency of the iinal heated coating, which'in the absence of this or equivalent material may be slightly,

hazy or translucent rather Ithan fully transparent.y

" Production of permanentcopies'requiresadditionally that the fusible material and the binder material be mutually compatible. A method of determining mutual compatibility is presentedin connection with Example l hereof. the melting-point -of the fusible material alone and of a mixture of the fusible material and binder material after heating somewhat above the melting point ofthe former. In this test, a substantial increase in the melting point of the mixture over that of the fusible material alone indicates that the components are compatible, Whereas a substantially unchanged melting-point indicates lack of compatibility.' As an example of the latter system, a mixtureof 7.2 parts of mannitol, melting Vat 16S-6 C., and 2.8 parts of polystyrene was heated at a temperature Well above the melting point of` the mannitol for 40 minutes and cooled, withno evidence of com-r patibility. The same result was obtained with adipic acid and polystyrene, and with adipic acid, mannitol and polystyrene. In these examples the melting point, determined as in Example l hereof, was 147-149 C. for the adipic acid alone as well as for the mixture of adipic acid and polystyrene; and for the adipic acid. and mannitol the melting point was 123-128" C. both with and without the polystyrene.' Sheet materials coated with such mixtures by methods equivalent to those employed in making. the products of. the present invention have been observed to be unstable, and to revert to the opaque form.

In essence, the method consists in determining 12 ethyl cellulose lms all provide suitable supportingbase sheet material for our novel copying-paper. Theyy may, be opaque, colored, orcoated with opacifying or color-3, ing agents. They may be non-transparent but infra-red..

. transmitting; one example of such'a structureisf-a red lcellophane film-dyed to non-transparency with an infra,-T red-transmitting dyestuff such as Ponsol jade green. 'They-l. may also be both transparent and infra-red-trausmitting.v Other ibrous and non-fibrous Webs, includiugcertain fabrics, are also suitable for one or another ofthe,Y structures herein described. However, it has been found that metalfoil and-other equivalent materials whichhavef high heat conductivity are unsuitable, and hence metal, t foil or equivalent supporting base materials arespecically excluded. Y l Additional examples of heat-sensitive copying-paper," made in accordance With the present invention will ,now be presented so thatvarious Aaspects of the invention, may be better understood and appreciated. The dispersions were produced by ball-milling to an extremely Vliner state of subdivision.4 f L Example 7 Parts; 40% dispersion of glycol monostearate in acetone 20 10% solution of cellulose acetate in 50 parts acetone,

40 parts toluene 20. Toluene 31 Diethyly phthalate j 1 s 2 parts ofcellulose acetate, and 1 part of diethyl phthalate at C. for 20 minutes with stirring produced, a blend whichfwhen cooled and tested as in Example'l was Vfound to melt` at S35-'54.5 C., whereas a mixture of 8 parts of glycolmonostearate and 1 part of diethyl: phthalate similarly ,treated and tested was found to .melt at S25-53.5 C., thus showing that the fusible materialand binder material Were compatible.

Example 8 Parts 40% dispersion of Opalwax in equal parts of ethyl acetate and heptane 25- 10% solution of ethyl cellulose in equal parts of ethyl acetate and xylol 35 Example 9 Parts Cornelowax No. 1469 (as dispersed solid par-l ticles) 14.4 Nitrocellulose (50 sec.) "4.11 Ethyl alcohol 81.5'

The dried coating, on colored paper, was surfacecoated with a very thin layer of a solution of .5 parts ethyl cellulose in 71.25 parts of toluene and 23.75 parts of heptane. The copying-paper was suitable for .back.

printing. Under test, the visible change, caused by transparentizing of the heat-sensitive layer, `occurred .at

' about 64C.

Acetone 80.75

The dispersion of waxy lparticles in binder solution was applied to black label paper as used in Example 9 and produced a useful heat-sensitive copying-paper which was permanently visibly changed by heating to approximately 64 C. Cornelowax No. 1469 is compatible with polyvinyl butyral las shown by an increase in melting point of the heat-blended mixture over that of the wax alone, and copies made of typewritten letters or similar graphic subject-matter with the copying-paper of this example are found to be permanent, the heat-trausparentized areas of the coating remaining transparent on prolonged aging at normal storage temperatures.

Other typical fusible materials, which in particulate form and combined with a compatible transparent binder of substantially the same reference index and for which the fused material has good wetting properties, are also useful in making our heat-sensitive copying-papers. Typical examples include ceryl alcohol, dicyclohexyl phthalate, dimethyl-m-phthalate, diphenyl, diphenyl phthalate, n-ethyl-paratoluene sulfonamide, octadecanol-l, O- and p-toluene sulfonamide (Santicizer 9), stearamide, and O-creasyl-p-toluene sulfonate.

What we clai-m is as follows:

1. The method of making a permanent fascimile copy of a graphic original having graphic representations highly absorptive of infra-red rays on a background relatively non-absorptive of infra-red rays, comprising placing a heat-sensitive copyright-paper, having on a supporting' base a visibly opaque heat-transparentizable layer comprising waxy particles fusible to a permanently transparent state and dispersed in a resinous film-forming organic binder infusible at the fusion temperature 'of said waxy particles, in heat-conductive relationship with said graphic original and then strongly and briey irradiating the graphic original with infra-red rays, thereby producing at the heat-transparentizable layer a heat pattern corresponding to said graphic representations andV of an intensity sufficient to transparentize the corresponding areas of said layer, and without ofsetting of fusible material.

2. T he method of making a permanent facsimile copy of a graphic original having graphic representations highly absorptive of infra-red rays on a background relatively non-absorptive of infra-red rays, comprising placing an infra-red-transmitting heat-sensitive copyingpaper, having on a supporting base a visibly opaque heat-transparentizable layer comprising waxy particles fusible to a permanently transparent state and dispersed in a resinous film-forming organic binder infusible at the fusion temperature of said waxy particles, against and in heat-conductive contact with said graphic original and then strongly and briey irradiating the graphic original with infra-red rays applied through the infra- '14 red-transmitting copying paper, thereby producing at the heat-transparentizable layer a. heat pattern corresponding to said graphic representations and of an intensity suicient to transparentize the corresponding areas of said layer, and without olsetting of fusible material.

3. The method of making a permanent facsimile copy of a graphic original having graphic representations highly absorptive of infra-red rays on a background relatively non-absorptive of infra-red rays, comprising placing a heat-sensitive copying-paper, having on a supporting base a visibly opaque heat-transparentizable layer comprising waxy particles fusible to a permanently transparent state and dispersed in a resinous film-forming organic binder infusible at the fusion temperature of said waxyparticles, the'waxy particles and organic binder being compatible as indicated by an increase in the melting-point of the heat-blended mixture over that of the waxy particles alone, in heat-conductive relationship with said graphic original and then strongly and briey irradiating the graphic original with infra-red rays, thereby producing at the heat-transparentizable layer a heat pattern corresponding to said graphic representations and of an intensity suicient to transparentize the corresponding areas of said layer, and without offsetting of fusible material.

4. The method of making a permanent facsimile copy of a graphic original having graphic representations highly absorptive of infra-red rays on a background relatively non-absorptive of infra-red rays, comprising placing a heat-sensitive copying-paper, having on a supporting base a visibly opaque heat-transparentizable layer comprising particles of a normally transparent stable organic fusible solid melting to a liquid without appreciable volatilization or decomposition at a temperature within the range of about 60-115 C. and distributed throughout a thin stratum of transparent lmforming organic binder which is infusible at the fusion temperature of said fusible solid, in heat-conductive relationship with said graphic original and then strongly and briefly irradiating the graphic original with infrared rays, thereby producing at the heat-transparentizable layer a heat pattern corresponding to said graphic representations and of an intensity suflicient to transparentize the corresponding areas of said layer, and without offsetting of fusible material; said fusible solid when in liquid form having good wetting properties toward Said organic binder, and said fusible solid and said binder being mutually compatible, as indicated by an increase in melting-point of the heat-blended mixture over that of the fusible solid alone, and having substantially the same refractive index.

References Cited in the le of this patent UNITED STATES PATENTS 2,188,590 Bjorksten lan. 30, 1940 2,669,038 Perry Ian. 6, 1942 2,668,126 Taylor Feb. 2, 1954 2,710,263 Clark June 7, 1955 

1. THE METHOD OF MAKING A PERMANENT FASCIMILE COPY OF A GRAPHIC ORIGINAL HAVING GRAPHIC REPRESENTATIONS HIGHLY ABSORPTIVE OF INFRA-RED RAYS ON A BACKGROUND RELATIVELY NON-ABSORPTIVE OF INFRA-RED RAYS, COMPRISING PLACING A HEAT-SENSITIVE COPYRIGHT-PAPER, HAVING ON A SUPPORTING BASE A VISIBLY OPAQUE HEAT-TRANSPARENTIZABLE LAYER COMPRISING WAXY PARTICLES FUSIBLE TO A PERMANENTLY TRANSPARENT STATE AND DISPERSED IN A RESINOUS FILM-FORMING ORGANIC BINDER INFUSIBLE AT THE FUSION TEMPERATURE OF SAID WAXY PARTICLES, IN HEAT-CONDUCTIVE RELATIONSHIP WITH SAID GRAPHIC ORIGINAL AND THEN STRONGLY AND BRIEFLY IRRADIATING THE GRAPHIC ORIGINAL WITH INFRA-RED RAYS, THEREBY PRODUCING AT THE HEAT-TRANSPARENTIZABLE LAYER OF A HEAT PATTERN CORRESPONDING TO SAID GRAPHIC REPRESENTATIONS AND OF AN INTENSITY SUFFICIENT TO TRANSPARENTIZE THE CORRESPONDING AREAS OF SAID LAYER, AND WITHOUT OFFSETTING OF FUSIBLE MATERIAL. 